The present invention is directed towards a method for fabricating an airfoil for water cooled gas turbine buckets. More particularly, the present invention is directed towards a method for fabricating airfoils of the type which include a central core surrounded by a plurality of airfoil coolant tubes located at spaced intervals around the periphery of the core and a composite outer skin encasing both the core and cooling tubes.
Turbine buckets of the foregoing type are known as open circuit liquid-cooled gas turbine buckets and are exemplified by U.S. Pat. Nos. 3,446,481 and 3,446,482, both issued to Kydd. A similar disclosure may be found in U.S. Pat. No. 4,137,619 issued to Adrian N. Beltran et al. The disclosure of these patents is incorporated herein by reference.
In typical prior art manufacturing methods, the central core is preformed to conform to the desired shape of the airfoil. Thereafter, a plurality of grooves which receive airfoil coolant tubes are formed about the outer periphery of the airfoil. These coolant tubes are independently fabricated, inspected and thereafter inserted into the grooves. Finally, an outer skin is welded or otherwise bonded to the central core to form the final airfoil.
One such process is described in U.S. Patent Application Ser. No. 749,719, filed Dec. 13, 1976, now U.S. Pat. No. 4,156,482 in the name of Anderson and assigned to the assignee of the instant invention. The Anderson application discloses a liquid-cooled gas turbine bucket comprised of a core having radially extending grooves of rectangular cross-section recessed into the core, tubes are fitted into these grooves and bonded in place by means of braze material filling in the space between the tubes and sides and bottom of the grooves. A protective skin is then bonded to the outer surface of the core to provide corrosion resistance. The skin is described as a composite of an inner skin which provides high thermal conductivity and an outer skin which provides protection from hot corrosion. The disclosure in the Anderson application of useful materials for preparation of the outer skin, the inner skin, the tubing and the bucket core is incorporated by reference.
The general method of incorporating an assembly to be brazed in a flexible envelope for the establishment within the envelope of a substantially non-oxidizing atmosphere and providing for force application to the envelope during brazing is disclosed in U.S. Pat. No. 3,091,846 to Henry. The disclosure therein is incorporated by reference.
In U.S. Pat. application Ser. No. 889,111, filed Mar. 22, 1978, now U.S. Pat. No. 4,185,369 in the name of Darrow et al and assigned to the assignee of the present invention, a method of fabricating an airfoil which more efficiently transports heat energy from the inside of the airfoil skin surface in contact with hot gas to the outer surface of the coolant tubes recessed in the bucket core is disclosed. In accordance with this method, the bucket is formed from a series of preformed solid components which are assembled, consolidated and then converted into a unified structure. In each of the several embodiments disclosed in the foregoing application, one of the preformed solid components is a flat bi-metallic sheet comprising an erosion-, corrosion-resistant layer and a layer of high thermal conductivity, these layers being joined by an optimized metallurgical bond. The disclosure of this application is incorporated herein by reference.